PHP- MySQL cronjob sending mail loop repeat -

in 1 of web application, sending daily mails users using cron job function via amazon ses. cronjob run in interval of 10 mins. process like

$sql-mysql_query("select * users send_date='2013-07-13' , alert_send=0"); while($row=mysql_fetch_array($sql)) {         // email id of user , compose meggage        // create new amazon request , send mail        // update alert_send=1  } 

if loop contains more counts ie suppose 500 mails, on next 10th min, cron job start , start sending mails. @ end users mail twice or thrice accordingly.

ie. if loop contains 500 datas

cron start @ 12:00 , fetch 500 datas , send emails. suppose send mails 120 10 mins.

cron b start @ 12:10 , fetch data 120 - 500. send mail.

by result, 121 th user mails cron , cron b.

i tried of limit query count. problem cant predict when each loop ends. ie take 4,2,5,6 or 10 secs send mail.

is there way avoid duplicating ? there way kill existing cronjob , start new ?

thanks in advance

everytime when cronjob start can kill cronjob running

like name:

pkill process-name 

or id:

kill 1234 

also can take @ blog use php code :

http://abhinavsingh.com/blog/2009/12/how-to-use-locks-in-php-cron-jobs-to-avoid-cron-overlaps/